Sleep deprivation has been shown to have mood-modulating effects in both normal subjects and patients with major depressive disorder, but the mechanism for these effects are unknown. Sleep deprivation also leads to sleepiness, which can impact significantly on performance, particularly in low-stimulus situations. Measurement of vigilance level could help to predict performance lapses, but currently available measures are either difficult to administer in performance settings or not highly reliable. The purposes of this study are to (1) characterize the effects of sleep deprivation on mood in normal subjects, and (2) assess the effectiveness of the startle response in measuring vigilance level. Subjects will undergo electrophysiological measures of regional brain function and emotion-modulated startle responses following a night of normal sleep and after one night of total sleep deprivation.